1. Field of the Invention
This invention relates to electric devices, and more particularly to power stealing circuits.
2. Description of Related Art
FIG. 1 shows a block diagram of electric device 40 for processing data signals received on signal input terminal 42. Device 40 is powered by power supply 44 providing a supply voltage on power input terminal 46 of device 40.
In some applications, it is inconvenient to power a device by a separate power supply. Such applications are provided with a power stealing circuit as shown in FIG. 2. Power stealing circuit 50 receives data signals on its signal input terminal 52 and provides them on its signal output terminal 54 which is coupled to signal input terminal 42 of device 40. Power stealing circuit 50 diverts (or "steals") some power from the data signals and provides that power on its power output terminal 60. Power output terminal 60 is coupled to power input terminal 46 of device 40. The application of FIG. 2 does not have a separate power supply, but device 40 is powered by the power "stolen" from signal input terminal 52.
FIG. 3 shows a block diagram of a system using the scheme of FIG. 2. Computer 66 sends data signals to device 68 via cable 70. Computer 66 has connector 72 which mates with connector 74 of cable 70. Connector 72 can thus be coupled directly to connector 74. It is sometimes desirable, however, to perform additional processing of the data signals from computer 66 to device 68. Interconnecting device 80 is plugged between computer 66 and cable 70. Connector 82 of interconnecting device 80 mates with connector 72 of computer 66 and connector 84 of interconnecting device 80 mates with connector 74 of cable 70. Interconnecting device 80 receives data signals on the leads (not shown) of connector 72, processes the signals and supplies them to the leads (not shown) of connector 74. Interconnecting device 80 does not have its own power supply, but is powered by the power "stolen" from the data signal on the leads of connector 72.
FIG. 4 shows a block diagram of interconnecting device 80. Leads 90a, 90b, 90c and others (not shown) from connector 82 are coupled to signal input terminals of respective power stealing circuits 94a, 94b, 94c. Signal output terminals 96a through 96c of respective power stealing circuits 94a through 94c are coupled to processing device 98. Power output terminals 100a-100c of respective power stealing circuits 94 are coupled to power input terminal 102 of processing device 98. Processing device 98 processes the input signals as required and provides them on its respective output terminals 104a-104c coupled to connector 84.
FIG. 5 shows a circuit diagram of prior art power stealing circuit 110. Signal input terminal 52 is coupled to signal output terminal 54 and to the anode of diode 116. The cathode of diode 116 is coupled to power output terminal 60. Reservoir capacitor 120 is coupled between power output terminal 60 and reference terminal 121 coupled to ground. The cathode of diode 122 is coupled to signal input terminal 52, and the anode is coupled to reference terminal 121.
When the voltage on signal input terminal 52 exceeds the voltage VDD on power output terminal 60 by a p-n junction barrier potential of diode 116 (about 0.6 V for silicon diodes), diode 116 conducts. Current flows through diode 116 to power output terminal 60 and charges reservoir capacitor 120.
When the voltage on signal input terminal 52 goes below the voltage on power output terminal 60, diode 116 does not conduct preventing the loss of current from power output terminal 60 to signal input terminal 52. Reservoir capacitor 120, which stores the charge, discharges providing, for a while, power on power output terminal 60.
Diode 122 protects power stealing circuit 110 and the circuit (not shown) connected to terminals 54 and 60 from noise such as negative spikes on signal input terminal 52. When the voltage on signal input terminal 52 goes below ground far enough to forward bias diode 122, current flows from ground to signal input terminal 52.
An object of the present invention is to provide an efficient power stealing circuit for generating a supply voltage from a source of data signals.